Wind turbine generators are normally connected to the grid by means of high-voltage switchgear. Associated with each wind turbine generator are auxiliary circuits powered by a low-voltage electricity supply, which is typically derived from a transformer connected to the high-voltage grid.
The auxiliary circuits include various protection arrangements for ensuring safe operation of the wind turbine generator, such as arc detection circuitry, switchgear protection relays and other safety relays, such as those manufactured by Pilz Automation Technology. In the event of a dangerous condition, such as a fire hazard, the auxiliary circuits can disconnect the wind turbine generator from the grid by opening the high-voltage switchgear.
When the voltage in the grid falls below a predetermined level, it is no longer possible for the grid to supply power to the auxiliary circuits. For this reason, wind turbine generators are normally provided with an auxiliary low-voltage power source which is arranged to supply the auxiliary circuits with power in the event that the voltage in the grid falls below its normal level. In practice, the auxiliary low-voltage power source is connected to the auxiliary circuits, and during normal operation the power source is itself supplied with power from the grid via the transformer.
The auxiliary low-voltage power source is typically in the form of an uninterruptable power source (UPS) which includes a rechargeable battery and associated circuitry.
The associated circuitry normally functions only when above a predetermined temperature, which is typically 0° C. For this reason, the UPS is normally supplied with a heater for maintaining the temperature of the associated circuitry above this level.
In the event of the grid voltage falling below its normal level, the auxiliary power source can supply power to the auxiliary circuits for only a limited time. When the voltage in the grid returns to its normal level, there can be a significant delay before the auxiliary circuits become fully functional. At normal temperatures, this delay can be approximately 20 seconds. However, at low temperatures, such as those which may be encountered by wind turbine generators located in cold climates, a further delay results from the need for the heater to raise the temperature of the UPS circuitry to its operational level. Depending on the local ambient temperature, the auxiliary circuits can remain unpowered for up to 1 hour. As a result, the main circuitry of the wind turbine generator will be unprotected during this time.
The Applicant has appreciated that it would be desirable to provide an arrangement which overcomes, or at least mitigates, this problem.